Many amateur SEO's may still consider the meta description to be a valid ranking factor, and, in a way, I agree!! But not in the same way...
Often, webmasters and those doing on-page SEO and content management for a website overlook meta descriptions, as their content doesn't directly influence rankings. This is foolish! As without them, Google is likely to use it's own decision on content. Often this is either irrelevant (prices, address details, product names etc) or it doesn't flatter your offering - perhaps just mentioning one of the many brands or services you offer (putting off those who are looking for anything else).
So, how do meta descriptions influence rankings?
If your meta description is compelling and sells the product or service that you have to offer better than your competitors – people will click-through to your page. If people visit your page and it features link-worthy content, offers valuable information and provides a solution – people will link to it!
Conclusion: Good meta description > more click-through's > more traffic > more links > better rankings!
Also, many SEO's believe that in some ways, your click-through rate and conversion rate influence rankings - and meta descriptions will have a significant impact on both of these as you will generate more traffic with a description that entices users to choose your listing (meaning more conversions).
Conclusion: Good meta description > more click-through's > more conversions > better rankings!
So, although indirectly, meta descriptions can = better rankings!